1. Field of the Invention
The present invention relates generally to packaging for an integrated circuit incorporating an improved substrate on which the integrated circuit is mounted and to an improved process for cooling integrated circuits that can be implemented with the improved substrate. More particularly, it relates to such a substrate and process which has an enhanced heat removal capability at a reasonable cost.
2. Description of the Prior Art
Substrates on which integrated circuits are mounted that are presently available do not provide all of the mechanical, electrical and thermal properties that are desired, particularly for high performance integrated circuits. Typically, the designer must sacrifice one property for another. For example, conventional ceramic substrates are very desirable from an electrical and mechanical standpoint, but they may not provide the thermal conductivity required for high performance integrated circuits. Molybdenum provides better thermal conductivity, but it is unfortunately an electrical conductor.
It is known to provide a heat pipe effect in porous media. Porous media heat pipes are a popular topic in space applications, since they do not rely on gravity to move the liquid around, but rather, the capillary forces created by the pores in the porous media move the liquid.
Recently, micro heat pipes have been demonstrated which move heat away from a hot source, such as an integrated circuit chip, in the lateral direction. Such heat pipes are manufactured by etching a small pattern of grooves in a flat piece of silicon. A cover is then sealed over the grooves while they are partially filled with a liquid. A heat source, such as the integrated circuit chip, at the center of the pattern generates vapor that moves away and through the pattern. At the perimeter of the pattern, the vapor condenses. The capillary forces in the grooves return the liquid back to the hot spot. Currently, this technology is limited to two dimensional, lateral heat movement.
At least one commercial heat pipe manufacturer, Thermacore, Lancaster, PA, has plans to introduce a conventional style heat pipe with an integral ceramic surface. The ceramic is not used to move the liquid with capillary forces, but it is used as a surface on which an integrated circuit could be mounted. The cavity of the heat pipe is essentially hollow. The cavity has the working fluid and a wick inside it. A disadvantage of this kind of heat pipe is cost, since the entire heat pipe could not be made from one piece of material.
Ceramics are used throughout electronic packaging as the conventional substrate on which integrated circuits are mounted. Their electrical insulating properties makes ceramics highly desirable substrates. The coefficient of thermal expansion for ceramics is reasonably close to that of silicon, the most common integrated circuit material. Typically however, the gas permeability of the ceramics used in electronics is close to zero, which would prevent the fluid movement required for the heat pipe phenomenon to occur.
Hermetic seals or coatings for ceramic substrates are also quite common. Ceramic surfaces have been coated by sputtering or electroless plating metals onto them. It is possible to coat ceramics with glass, since ceramics can take the very high temperatures of the glass firing operation.
Thus, while the heat pipe art and the integrated circuit packaging art are both well-developed, a need remains for a high performance integrated circuit package that provides suitable mechanical, electrical and thermal properties at a reasonable cost.